Utilizing immunocytochemical techniques for the localization of cyclic nucleotides, we have been able to demonstrate in both rat liver and kidney changes in localization of cyclic GMP without any change in the tissue level of the nucleotide. This is seen during the process of liver regeneration, or in kidney, after the administration of parathyroid hormone. The localization of cyclic GMP to plasma membrane and nuclear elements suggests a role for the nucleotide in membrane transport and nucleus-directed events, respectively. BIBLIOGRAPHIC REFERENCES: Ong, S. H., A. L. Steiner, "Localization of Cyclic AMP and Cyclic GMP in Cardiac and Skeletal Muscle: Immunocytochemical Demonstration", Science, 195: 183, (1977). Stowe, N. W., T. H. Whitley, R. L. Ney, and A. L. Steiner, "Changes in Rat Adrenal Cyclic Nucleotides During Normal and Neoplastic Growth", Biochemistry Biophys. ACTA, 497: 690, (1977).